1. Field of the Invention
The present invention relates generally to document reproduction apparatus and more particularly to an improved circuit for use in such an apparatus to provide compensation for variations in the luminosity of documents and nonuniformities in the reproduction scanning apparatus.
2. Description of the Prior Art
Variations in the background level of signals obtained from a document scanner occur both because of variations in the luminosity of different document papers, or colors of the paper, and because of nonuniformities in the scanning system. Typical sources of variation in a scanning system include decreased sensitivity at the document edges because of the increased distance to the illumination source and the increased angle of reflection, variations in the scanner optics, such as vignetting, and aging of the illumination source and photodetector. To avoid the consequences of such variations, white level compensation is usually employed.
One prior art approach filters a portion of the video signal from the scanner in a long time constant circuit, called a white follower. The video signal and the filtered signal are compared in a discriminator which produces an output consisting of the differences in the signals. The resulting signal contains only the rapid signal variations. One difficulty with this approach is that it is unable to reproduce large shaded areas of a document. A second problem is that printing at the edge of shaded areas, such as on an address label, is lost.
A prior art approach to solve the second problem is disclosed in the IBM Technical Disclosure Bulletin, Volume 13, No. 11, April 1971. This approach includes a white follower having two time constant configurations. Normally, the regular long time constant configuration is used; however, when black is encountered, the much faster time constant configuration is activated. Although tending to solve the edge distortion problem, this approach tends to aggravate the first mentioned problem.
A system to compensate for gain variations between the different elements of the photodiode array of a scanner is disclosed in the U.S. Patent to McNeil et al, U.S. Pat. No. 3,800,079. This system operates in two modes. In the first mode, a standard is scanned and the difference between the output of each photodiode and a reference is stored. In the second mode the recovered stored difference is added to the reference producing ideally the original diode output. This latter signal is first subtracted from the current output resulting in a representation of the information being scanned. Finally that same signal is divided into the result of the aforementioned operation to generate the calibrated signal. In addition to requiring a complex system having two separate modes of operation, the system calibration is not performed on the document to be copied; thus, the system is unable to compensate for differences in document luminosity. Were calibration attempted on the document to be copied, the system would be unable to differentiate between the background and data levels and would result in an inaccurate calibration.